A monocrystalline crystal is a crystalline solid in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. Crystal growing furnaces may be used to grow monocrystalline crystals using the Czochralski (CZ) process. Similarly, multicrystalline crystals may be formed in multicrystalline solidification furnaces. These crystals are usually silicon. The heart of the furnace is the stainless steel chambers where the silicon is melted and the crystal is grown. The crystallization growth process takes place under a vacuum. Typically, Argon is introduced into the top of the chambers to flush out any impurities. Connected to the bottom of the chambers are vacuum lines which draw the purge gas out of the chambers and maintain the desired vacuum level.
During the crystallization process, a quartz crucible is commonly used to hold the silicon inside the furnace. An undesired byproduct of the process is silicon monoxide (SiO) which is formed as some of the oxygen is dissolved out of the quartz crucible at the high temperatures. This SiO has the consistency of a fine powder. SiO is also an unstable substance which will react with oxygen when exposed to air to form silicon dioxide (SiO2). During this reaction, heat is generated.
The vacuum pumps which are used for this application may be one of several types including, piston, liquid ring, or dry pumps. Many of the pumps today also use blowers to increase the pumping speed. During the process, when the SiO travels down the vacuum lines and reaches the vacuum pumps, it can cause damage to dry pumps or blowers which have small clearances between the moving parts. In piston or liquid ring pumps, the SiO will become trapped in the oil, and will force the oil to be changed frequently. Therefore, a filter is almost a necessary component for a crystal growing furnace to remove the SiO.
Currently available on the market are several different filter assemblies. One is a simple pleated paper element in a stainless steel housing. This is a low cost solution, but has the following drawbacks. First, cleaning or replacing the elements is a dirty job because the housing must be opened. Second, the SiO reaction into SiO2 can burn a hole in the paper element if air is introduced quickly. This breach of the element will lead to damage to the vacuum pump.
Another conventional filter assembly is a backpulsing filter unit which uses pleated elements inside a housing. There are usually four pleated elements. Instead of opening the housing to clean the elements, a burst of argon is introduced inside the filter element to flex the element and knock the SiO powder off of the outside of the element. This solution has the following drawbacks: (i) the pleated filter elements are 8-10 inches in diameter, so a filter incorporating four (4) elements will require a large housing approximately 2.5-3 feet in diameter; (ii) a separate tank is required to contain the volume of argon necessary to generate the pressure pulse to clean the elements; (iii) vacuum valves are required on either side of the filter housing so that it can be isolated during cleaning; and (iv) interlocks are required to verify that the vacuum valves are closed before the argon pulse is introduced.
Yet another conventional filter assembly includes a back pulsing unit and a multitude of non-pleated stainless steel filter elements. Rather than using argon as in the conventional filter described above, a burst of room air is introduced inside the filter housing to knock the SiO powder off of the outside of the filter element. First, the filter housing is drawn under a vacuum and the valve on either side of the housing is closed to isolate the housing. Then, a valve at the top of the filter housing is quickly opened to allow air to rush into the housing. The air flow runs across the elements knocking the particulate off of the outer surfaces of the element. This assembly has the following drawbacks: (i) the assembly can be cleaned only after a completed run because the filter housing must be isolated during cleaning; (ii) the use of room air can potentially cause a rapid and undesirable reaction between the oxygen in the air and the SiO particulate; and (iii) the back pulsing is often not forceful enough to completely clean the layer of SiO off of the elements.
It is desired to develop a self cleaning vacuum filter assembly having less components and a smaller filter housing, which will not require such a large space in a user facility.